The invention relates to a zero-contact switch electronic.
A switch is known from the publication "IC for Proximity Detector FM 303," May 9, 1988, and produced by the company Favag SA Microelectronics, Switzerland. In that document, the sensor signal from an externally tunable oscillator is fed to an evaluation stage which controls two driver stages operating in a push-pull mode. In addition, the evaluation stage switches an LED driver with a separate LED output to which an LED is directly connected to display the switch state. A short-circuit interrogation device shuts off only the two driver stages at intervals to protect the entire circuit when a short circuit occurs. A disadvantage of this arrangement is that the LED switched by the evaluation stage, still displays the control signal for the driver stages even during the short circuit and provides no information about the short circuit which has actually occurred.
The publication Siemens IS-Specification S594 of Apr. 22, 1986 also discloses a switch constructed using the "TCA 505" IC of "Siemens AG." In this IC, IC-internal output transistors with open collectors or open emitters are controlled by the two driver stages. It is known from the description of these ICs that an LED can be connected between the supply voltage and an open emitter to display the switch state. This LED indicates by gently pulsing the presence of a short circuit in the load circuit, since both driver stages are switched on briefly by the short circuit interrogation device to interrogate the short circuit. As a result, there is a danger that confusion could occur at high switching frequencies of the switch. The IC used for this purpose requires a total of four driver stage outputs which must always be externally switched to the IC to ensure the function and is therefore expensive. In addition, the base voltage of the internal output transistors is limited so that an additional IC connection is required and the output transistors, are coupled together inside the IC.
From DE-OS 37 31 774, the block diagram of a switch is known in which a common LED is used to display the switch state and to indicate a short circuit. A switching stage switches both the LED and the load switch and, in the event of a short circuit, is controlled by a short circuit interrogation device in such a manner that a short circuit is indicated by flashing. Here again, there is no independently tappable signal for an unambiguous indication of the short circuit, so that a clear distinction is not always ensured between a short circuit and the display of the switch state.
A switch constructed with discrete elements is known from DE-OS 34 41 403, which has a single LED to display the switch state and a short circuit which flashes at a certain frequency in the event of a short circuit. Because it is composed of discrete elements, it can provide distinctive indications for the switch state, and the case of a short circuit and customer-specific requests can be met; but in switches according to the species which use an IC, the special requirements of a customer can only be met by costly external switching of the IC since the manufacture of differently integrated circuits would be much too expensive.